This invention relates to plasma sources and, more particularly, to sources of soft X-ray or extreme ultraviolet photons, or sources of neutrons, wherein high power production of photons or neutrons is achieved by electrostatic acceleration of ions toward a plasma discharge region, neutralization of the ions to avoid space charge repulsion as the discharge region is approached and the application of a heating current through the central plasma in order to raise its temperature and density.
Soft X-ray and extreme ultraviolet photons can be generated in a hot plasma. The wavelength of the photons is determined by the mixture of ionization states present, with generally shorter wavelength photons being produced by the radiation of higher ionization states within the plasma. An example relevant to lithography is the xenon plasma that contains states Xe10+, Xe11+ and Xe12+ and radiates strongly in the 10-15 nanometer (nm) band of the spectrum. Within this band, the 13.5 nanometer wavelength is considered to be the optimum for lithography because it can be reflected with up to 70% efficiency by molybdenum-silicon multilayer mirrors in a combination that re-images the pattern of a semiconductor circuit from a mask onto a silicon wafer.
Several approaches to the generation of these energetic photons have been researched in recent years. The plasma has been heated by laser pulses in the so-called laser-produced-plasma (LPP) method. Also, the plasma has been heated directly by the passage of a pulsed electric current in a variety of discharge-produced plasma (DPP) photon sources. These include the capillary discharge, the dense plasma focus and the Z-pinch. It is believed that a viable 13.5 nm source for commercial, high throughput lithography will be required to emit approximately 100 watts of photon power into 2 steradians in a 2% fractional band at 13.5 nm, from a roughly spherical source of diameter less than 1.5 millimeters. In xenon, which is the most efficient 13.5 nm radiator (of room temperature gaseous elements), the 2% fractional band is produced at an electrical efficiency of approximately 0.5% into 2xcfx80 steradians in DPP sources and up to 1% into 2xcfx80 steradians in LPP sources relative to laser power absorbed. For the lithography source, a plasma power of 30-60 kilowatts (kW) is therefore required. Other requirements are for precise plasma positioning, to provide uniform illumination, and a repetition frequency greater than 6 kHz.
In the prior art, the plasma has been positioned, in the case of a laser produced plasma, by the intersection of a stabilized beam of liquid xenon with a focused laser beam. The size and positional stability of the resulting plasma are compatible with the application, but with laser efficiencies of only 4% for the pulsed lasers of interest, an electrical input power of 750 kW to 1.5 megawatts is likely to be needed in order to generate 100 watts of 13.5 nm photons, making the economics of the LPP source very unfavorable.
By supplying electrical energy directly to the plasma, the DPP source can, in principle, have a power input not much greater than the 30-60 kW plasma power. However, in prior art discharges, the plasma has, with the exception of the dense plasma focus, been too large in at least one dimension, and the dense plasma focus itself depends on a closely positioned electrode, only a few millimeters distant from the plasma, to create a small, positionally stable plasma focus. There are limits to the plasma power that can be generated in such close proximity to a solid electrode, presenting a difficult scaling challenge for the dense plasma focus source.
Pending application Ser. No. 09/815,633 filed Mar. 23, 2001 discloses a new photon source, referred to herein as the astron source, wherein energy and material are fed into a plasma at a central location via numerous energetic neutral beams. In this source, a relatively large separation has been achieved between the plasma and the nearest solid surface. The astron source also has a distributed electrode which exhibits low current density and anticipated longer life. Although this approach has enabled the generation of a hot plasma that emits extreme ultraviolet photons and is capable in principle of being scaled to 30-60 kW plasma power, it depends on a high acceleration efficiency for the neutral beam particles. To date, only 20% efficiency has been measured, and improvements in acceleration efficiency are required in order to give this photon source a good electrical efficiency.
Accordingly, there is a need for improved methods and apparatus for generating soft X-ray or extreme ultraviolet photons.
According to a first aspect of the invention, a source of photons is provided. The source of photons comprises a housing that defines a discharge chamber, a first group of ion beam sources directed toward a plasma discharge region in the discharge chamber, the first group of ion beam sources comprising a first electrode and an inner shell that at least partially encloses the plasma discharge region, and a second electrode spaced from the plasma discharge region. The source of photons further comprises a first power supply for energizing the first group of ion beam sources to electrostatically accelerate, from the first group of ion beam sources toward the plasma discharge region, ion beams which are at least partially neutralized before they enter the plasma discharge region, and a second power supply coupled between the first and second electrodes for delivering a heating current to the plasma discharge region. The ion beams and the heating current form a hot plasma that radiates photons.
In some embodiments, the ion beams and the heating current are both pulsed, and the pulsed ion beams precede the pulsed heating current. The ion beams may be at least partially neutralized by resonant charge exchange.
The radiated photons may be in the soft X-ray or extreme ultraviolet wavelength range. In some embodiments, the ion beams comprise xenon ions and the radiated photons have wavelengths in a range of about 10-15 nanometers. The ion beams may comprise ions of a working gas selected from the group consisting of xenon, hydrogen, lithium, helium, nitrogen, oxygen, neon, argon and krypton.
In some embodiments, the first electrode comprises a first hollow ring electrode. The first power supply may be connected between the first hollow ring electrode and the inner shell. The second power supply may be connected between the first hollow ring electrode and the second electrode.
In some embodiments, the source of photons further comprises a second group of ion sources. The second group of ion sources may comprise a second hollow ring electrode and the inner shell. The first power supply may have a first terminal connected to the first and second hollow ring electrodes and a second terminal connected to the inner shell. The second power supply may be connected between the first and second hollow ring electrodes.
In some embodiments, the second electrode comprises a cup electrode. The cup electrode may be coupled to the plasma discharge region through a hole in the inner shell. In some embodiments, the source of photons may further comprise a ring electrode mounted within the cup electrode and a third power supply coupled between the ring electrode and the cup electrode.
In some embodiments, the inner shell may be divided into a first shell portion corresponding to the first hollow ring electrode and a second shell portion corresponding to the second hollow ring electrode. The first and second shell portions may be connected by a resistor having a value that is large in comparison with the impedance of the plasma during delivery of the heating current.
In some embodiments, the second electrode may comprise a structure defining an aperture for emission of a photon beam from the plasma discharge region.
According to a further aspect of the invention, a system for generating photons is provided. The system comprises a housing defining a discharge chamber, a first group of ion beam sources directed toward a plasma discharge region in the discharge chamber, the first group of ion beam sources comprising a first electrode and an inner shell that at least partially encloses the plasma discharge region, and a second electrode spaced from the plasma discharge region. The system further comprises a first power supply for energizing the first group of ion beam sources to accelerate, from the first group of ion beam sources toward the plasma discharge region, beams of ions of a working gas, wherein the ions are at least partially neutralized before they enter the plasma discharge region, a second power supply coupled between the first and second electrodes for delivering a heating current to the plasma discharge region, a gas source for supplying the working gas to the discharge chamber, and a vacuum system for controlling the pressure of the working gas in the discharge chamber.
According to another aspect of the invention, the apparatus described herein may be used for the production of neutrons.